What is the Origin of Free Androgen Availability?

The concept emerged from the recognition that total hormone concentration alone was insufficient to explain clinical symptoms, leading to the characterization of hormone-binding proteins in the mid-20th century. The term emphasizes the critical distinction between the total circulating hormone and the fraction that is functionally available to the tissues. It is a cornerstone of precision endocrinology.

What is the Mechanism of Free Androgen Availability?

Androgens are synthesized and released into the bloodstream where they rapidly bind to carrier proteins, with SHBG binding with high affinity and albumin with lower affinity. The free fraction is determined by the total concentration of the androgen and the concentration and binding affinity of SHBG. Any alteration in SHBG production, often influenced by thyroid status or insulin levels, directly impacts the percentage of free androgen available to modulate gene expression and drive anabolic and metabolic processes.

Free Androgen Availability ∞ Area

Free Androgen Availability

Meaning

The fraction of circulating androgen hormones, primarily testosterone and dihydrotestosterone (DHT), that are not bound to plasma proteins, most notably Sex Hormone-Binding Globulin (SHBG) and albumin. This unbound fraction is the biologically active pool of hormone, capable of diffusing across cell membranes to interact with intracellular androgen receptors and exert its physiological effects. Clinically, this measurement provides a more accurate assessment of a patient’s true hormonal status and target tissue exposure than total hormone levels.